1. Field of the Invention
The present invention relates to a method of image processing and, more particularly, to a system of an image noise measurement and the method thereof.
2. Description of Related Art
Generally, TV signals are likely to suffer from the interference in transmission to thus have noises. To reduce the interference of the noises, a noise reduction is typically provided in a display section. However, the noise reduction in either spatial or temporal domain possibly produces various problems. Typically, the noise reduction is performed by first analyzing the noise levels of input images and then taking various noise reduction processes according to the noise levels analyzed.
U.S. Pat. No. 5,844,627 granted to May, et al. for a “Structure and method for reducing spatial noise” describes a method for spatial noise reduction, which first analyzes the spatial frequency components and then suppresses the possible bands with noises. However, the method for spatial noise reduction cannot completely separate the noises from the video components in space, and thus the side effect of blurs easily present in the video. U.S. Pat. No. 6,259,489 granted to Flannaghan, et al. for a “Video noise reducer” describes a method for temporal noise reduction, where the pixels of a still picture at different time on a same spatial position are taken a mean along a temporal axis if the noises are uncorrelated in the temporal axis and have a mean of zero. Accordingly, the reduced noise variance and the video with a lower noise level are achieved. However, the temporal noise reduction, which obtains the reduced noises without losing the spatial definition on the still picture, has to operate with detection of motion object occurred in the video to thereby avoid averaging the samples at different spatial positions and producing a motion blur or residual.
Generally, a viewer shows high tolerance in the side-effects caused by the noise reduction when the noise is at a high level, but the viewer shows relatively low tolerance in the side-effects when the noise is at a low level. Since the unacceptable detects are produced when a strong noise reduction and filtering method is applied to a low-noise video signal or the insufficient noise reduction on a high-noise video signal presents when a weak noise reduction and filtering method is applied, an accurate noise level measurement is required for an input video signal. Namely, an appropriate noise reduction and filtering strength is required for a good noise processing.
In order to accurately measure the noise level in the input video signal, U.S. Pat. No. 5,657,401 granted to Choi for a “Method for driving a matrix liquid crystal display panel with reduced cross-talk and improved brightness ratio” compares the sum of temporal absolute differences with a set of thresholds. When the sum locates in the upper and lower boundaries of the set, an accumulator is increased by one, and subsequently it is determined whether a total number of pixels in a predetermined interval is equal to an expected value. When the total number of pixels in the predetermined interval does not equal to the expected value, the set of thresholds is adjusted to thereby respond the noise level in the video signal. However, a picture contains the different proportions of motion areas, and accordingly the expected value cannot be predetermined easily and the noise level measurement can be easily affected by the number of pixels corresponding to the motion areas in the frame.
To overcome the aforementioned problem, U.S. Pat. No. 6,307,888 granted to Le Clerc for a “Method for estimating the noise level in a video sequence” uses the measured motion information to divide a signal into still and motion blocks. The still and the motion blocks are operated (such as calculating the sum of temporal absolute differences) with corresponding positions (still) and corresponding motion compensation blocks (motion) to find the noise estimates of the still and the motion blocks respectively, and subsequently the noise estimates of the still and the motion blocks are mixed to thereby obtain a final noise estimate. Such a manner requires an accurate motion estimation to thus measure the accurate noise levels in the motion blocks. However, a typical TV display system does not contain a motion estimation and compensation operation.
US Patent Publication No. 2006/0221252 for a “Reliability estimation of temporal noise estimation” converts a distribution of the temporal local difference into a characteristics value and compares the characteristics value to a threshold corresponding to an ideal distribution to accordingly determine to remain or discard the noise level of the current frame. The different motion degrees generally affect the distribution of the temporal local difference. However, the number of motion pixels present in the video signal is different, and the difference of motion time produced in the video signal is different. Accordingly, the distribution of the temporal local difference is gradually changed with the different motions, which increases the difficulty of finally determining to remain or discard the threshold.
Therefore, it is desirable to provide an improved image noise measurement system and method to mitigate and/or obviate the aforementioned problems.